It is recognized that the quenching properties of coolants, particularly those used in cooling aluminum alloy slabs from high temperatures during casting, are very dependent on the coolant composition, including the presence of small amounts of contaminants. This is particularly important when such coolants are to be used above the "film boiling temperature," since the effectiveness of cooling can be rapidly altered by slight changes to the film boiling temperature itself.
Various techniques have been used to measure the quenchant properties of coolants in the past. For example, U.S. Pat. No. 5,178,463 (Berry et al) describes the use of a flame-heated plate on which the quenchant to be tested is sprayed, the temperature gradient is measured, and the results are used to determine quenchant properties.
U.S. Pat. No. 4,756,357 (Banniger et al) describes an electrically-heated plate located within a casting mould where the flowing coolant passes the plate and the heat loss is measured. The signal is subsequently used for controlling the coolant flow or gas content.
U.S. Pat. No. 4,412,752 which issued on Nov. 1, 1983 to Cellitti et al., discloses a method and apparatus for determining the cooling characteristics of a quenching medium. The apparatus includes a temperature probe and a temperature sensor affixed to the probe. The probe and sensor are heated to a predetermined temperature and then immersed into a quenching medium. The quench rate is monitored to determine whether the quench rate is within predetermined limits at predetermined temperatures. However, the probe and affixed sensor are not suitable for rapid testing cycles of a kind required if the testing method is to be applied within a feedback system typical of industrial processes.
U.S. Pat. No. 5,289,867 which issued on Mar. 1, 1994 to Barker et al., discloses a method and apparatus for adjusting coolant flow rates in casting machines. The system is automated by using a stored set of cooling flow rate targets that are computed based on historical data. The historical data represents the combinations of target flowrates that yield desired product properties. Thus, the system is based on the automation of processes that an experienced operator might follow. This requires a complex database of historical data and programming based on the information.
U.S. Pat. No. 4,722,611 which issued on Feb. 2, 1988 to Hultgren, discloses an apparatus for testing quench-cooling properties of a liquid quenchant. The apparatus includes a thermistor forming one arm of a bridge circuit. The thermistor acts as both a heater and a temperature sensor. However, if the heating function is terminated, the temperature sensing also terminates, so the apparatus is not well suited for measuring quenching properties of liquids. The apparatus really measures the power dissipation of the thermistor and does not directly measure the temperature drop of a body in contact with a liquid quenchant.
U.S. Pat. No. 4,106,331 which issued on Aug. 15, 1978 to Bunton et al., discloses a method for detecting the contamination of a liquid such as a quench oil. The "probe" employed by Bunton et al. is a stainless steel slug having an embedded thermocouple and the slug is heated in a furnace. Clearly, this arrangement is not intended for rapid testing of quenching fluids because heating of the slug in a furnace is required and this is necessarily a slow step.
Both these methods employ fairly large heated elements and require the use of substantial quantities of flowing coolant.
A paper entitled "The Effect of Cooling Water on Aluminum Ingot Casting" by H. Yu, Light Metals 1994. The Metallurgical Society of the AIME, pp. 779-786, describes a small cylindrical element containing a thermocouple which is preheated in a furnace then immersed in a sample of coolant. While this device is more compact than the devices required for the preceding methods, a separate preheating furnace is required, and the thermal response is limited because of the slow response time of the embedded thermocouple.
There is a need, therefore, for a compact and fast response system for measuring coolant quenchant properties, including properties above the film boiling temperature, that can also serve as part of a control system for altering the coolant flow or properties in response to changes in such quenchant properties.